A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to ensure the proper operation of the IC, an accurate positioning of the substrate during the exposure process is desired. As such, a lithographic apparatus is typically equipped with one or more position measurement systems to determine a position of an object table (to which the substrate is mounted) relative to the patterning device. As an example, a lithographic apparatus can be provided with an incremental position measurement system, e.g. an encoder based measurement system or an interferometer measurement system, for measuring the position of an object table relative to a projection system of the apparatus. A further, similar measurement system can be provided to determine a position of the patterning device relative to the projection system. Typically, an incremental position measurement system provides in a periodic measurement signal (e.g. having a period corresponding to a period of a grating of the measurement system) that needs to be referenced or zeroed in order to obtain an absolute position measurement. Such referencing or zeroing may require the object table to move to a particular designated reference position and/or may require adjustments to the measurement system. In case the referencing requires a particular positioning of the object table, this may result in a throughput penalty for the apparatus.